x86: inline the 'rep movs' in user copies for the FSRM case

This does the same thing for the user copies as commit 0db7058e
("x86/clear_user: Make it faster") did for clear_user().  In other
words, it inlines the "rep movs" case when X86_FEATURE_FSRM is set,
avoiding the function call entirely.

In order to do that, it makes the calling convention for the out-of-line
case ("copy_user_generic_unrolled") match the 'rep movs' calling
convention, although it does also end up clobbering a number of
additional registers.

Also, to simplify code sharing in the low-level assembly with the
__copy_user_nocache() function (that uses the normal C calling
convention), we end up with a kind of mixed return value for the
low-level asm code: it will return the result in both %rcx (to work as
an alternative for the 'rep movs' case), _and_ in %rax (for the nocache
case).

We could avoid this by wrapping __copy_user_nocache() callers in an
inline asm, but since the cost is just an extra register copy, it's
probably not worth it.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>